The construction industry faces dual challenges of developing sustainable materials while advancing innovative manufacturing technologies. With increasing scarcity of fly ash due to coal power plant closures, alternative precursors are needed. Municipal solid waste incineration bottom ash (MSWI BA), currently landfilled in large quantities, presents an opportunity to address both material scarcity and waste management challenges. However, 3D concrete printing technology requires materials with precisely controlled rheological properties balancing conflicting requirements: sufficient fluidity for pumping and extrusion, yet adequate stiffness for shape retention. Developing mixtures meeting these stringent printability requirements while incorporating waste materials represents a significant challenge.

This study aims to investigate the impact of municipal solid waste incineration bottom ash (MSWI BA) as a sustainable alternative to fly ash (FA) on the printability and early-age behaviour of slag-based alkali-activated materials for 3D printing applications. The research followed a systematic three-phase approach: mix development and optimization, investigation of MSWI BA effects on rheology and early-age reaction kinetics, and environmental impact assessment through life cycle analysis.

The first phase involved development of two 3D printable slag-based AAM mortars through systematic optimization for printability. The optimization process evaluated buildability through slump tests, flowability through slump flow measurements, extrudability through mini-extrusion tests, setting time through Vicat testing, and mechanical strength development through compressive and flexural tests. The optimized reference mix composition comprised 80% slag and 20% FA with water-to-binder ratio of 0.38, alkali content (Na₂O/b) of 5%, activator modulus (SiO₂/Na₂O) of 0.5, and sand-to-binder ratio of 1.5. The target mix comprised 80% slag and 20% MSWI BA, maintaining identical parameters except water-to-binder ratio increased to 0.40 to achieve comparable workability, necessitated by BA's angular morphology and finer particle size. Both mixes met all printability requirements and exceeded mechanical strength targets.

The second phase revealed fundamental mechanisms governing the observed behavioural differences. Yield stress evolution via slugs test showed the FA-based mix exhibited rapid structuration with brittle discontinuity at 80 minutes, while the BA-based mix maintained slug formation throughout 140 minutes, confirming extended printable window. Pore solution analysis through revealed the BA system consistently showed lower elemental concentrations and slower consumption rates of key elements (Na, Si, Ca, Mg), indicating reduced precursor dissolution. This was attributed to dilution effects from higher water content and, more significantly, heavy metals (Cr, Zn, Pb) from MSWI BA forming hydroxide precipitates on slag surfaces, hindering slag dissolution. Fourier transform infrared spectroscopy confirmed slower reaction kinetics, showing persistent low-polymerized silicate oligomers in the BA system that sustained electrostatic repulsion between particles, delaying percolated network formation and extending printability window.

Environmental assessment demonstrated both the developed slag-based AAM mixes achieved approximately 68% reduction in shadow costs compared to 3D printable ordinary Portland cement mortar, with MSWI BA contributing less than 5% to total environmental impact. This research demonstrates that MSWI BA can effectively replace fly ash at 20% binder content in 3D printable slag-based AAMs, providing extended printable window without compromising performance.